Martinez left Sunday's 5-1 win at San Francisco and was replaced by Jason Varitek in the bottom of the fourth. Martinez took two foul tips off his thumb, which swelled up so much he couldn't stay in the game. An X-ray revealed the fracture.
The Red Sox did not know whether Martinez will need to go on the disabled list — Martinez didn't think so. He will be re-evaluated on Monday's off-day.
"It's not a given that he'll go on the DL but it's not a given that he'll be OK, either," manager Terry Francona said. "We'll get him examined tomorrow and we'll see where we go from there."
Boston is already missing star second baseman Dustin Pedroia, who recently broke his left foot. Clay Buchholz, a 10-game winner, left his start Saturday after one inning because of a hyperextended left knee that he hurt running the bases.
That made for three injuries in three days against the Giants.
"We definitely want to get out of here," Martinez said.